Combination of a substrate and a wafer

ABSTRACT

The invention pertains to a combination of a substrate and a wafer, wherein the substrate and the wafer are arranged parallel to one another and bonded together with the aid of an adhesive layer situated between the substrate and the wafer, and wherein the adhesive is chosen such that its adhesive properties are neutralized or at least diminished when a predetermined temperature is exceeded. According to the invention, the adhesive layer is only applied annularly between the substrate and the wafer in the edge region of the wafer.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.13/464,187, filed May 4, 2012, which is a divisional of U.S. applicationSer. No. 12/916,751, filed Nov. 1, 2010, now issued as U.S. Pat. No.8,349,701, which is a divisional of U.S. application Ser. No.11/610,750, filed Dec. 14, 2006, now issued as U.S. Pat. No. 7,910,454,which claims priority from DE 102006000687.9, filed Jan. 3, 2006, saidpatent applications hereby fully incorporated herein by reference.

FIELD OF THE INVENTION

The invention pertains to a combination of a substrate and a wafer, adevice for separating a combination of a wafer and a substrate as wellas a method for handling a substrate and a wafer.

BACKGROUND OF THE INVENTION

In the semiconductor industry, it is necessary to subject a wafer to arethinning process after its structuring in order to reduce the axial orvertical dimensions and to realize electrical and thermal improvementsof the finished product. In the rethinning process, the non-structuredside of the wafer is removed by means of mechanical grinding methods.The electrically active layer and, if applicable, any contactingterminals provided on the wafer surface that are usually realized in theform of small soldering globules (pumps) remain untouched.

Since the rethinning process is usually realized by grinding the wafermechanically as mentioned above, it is necessary to fix the wafer on astable substrate in order to stabilize the wafer during the grindingprocess. This type of stabilization is particularly important for analready rethinned wafer because a wafer of this type only has a veryslight mechanical stability.

In order to increase the mechanical stability of the wafer, it needs tobe initially fixed on a substrate. However, the substrate needs to beremoved again after the rethinning process is completed, namely withoutdamaging the sensitive rethinned wafer during this process.

Numerous methods and devices are used in practical applications forproducing this temporary connection between a wafer and a substrate. Inall these methods, it is attempted to reuse the substrate for costreasons. If silicone is used as the base material for the electriccircuits, a silicon wafer is also used as the substrate in practicalapplications due to the favorable costs and the identical coefficient ofthermal expansion. Substrates of glass, sapphire or other materials areused if the wafers consist of a different material, for example, GaAs.

One widely used method for fixing a wafer on a glass substrate consistsof bonding the glass substrate to the wafer over a large surface. Theadhesive used has the peculiar property of losing its adhesiveproperties once a certain temperature is exceeded. In order to separatethe wafer from the glass substrate, energy is applied to the glass waferby means of a laser such that the adhesive is dissolved. The wafer andthe glass substrate can then be easily separated from one another. Thedisadvantage of this known method in practical applications can be seenin that the adhesive frequently is already destroyed by the temperaturesoccurring during the rethinning process such that the wafer alreadyseparates from its substrate during the rethinning process. This resultsin the destruction of the wafers.

Other known methods utilize films that are provided with an adhesivelayer. This adhesive also loses its adhesive properties when a certaintemperature is exceeded. Analogous to the previously described method,the high temperatures occurring during the rethinning process mayalready cause the connection between the substrate and the wafer toseparate.

Adhesives that only lose their adhesive properties at temperatureshigher than those occurring during the rethinning process cannot be usedbecause the temperatures required for neutralizing the adhesive woulddestroy the structures of the wafer.

SUMMARY OF THE INVENTION

The invention is based on the objective of proposing a combination of asubstrate and a wafer, in which the connection between the substrate andthe wafer withstands the temperatures occurring during a rethinningprocess without damages and can be separated with simple means after therethinning process is completed without damaging the structured wafer.Other objectives of the invention consist of proposing a device forseparating the combination of a wafer and a substrate, as well as amethod for handling a wafer and a substrate.

This objective is attained with a combination of a substrate and a waferwith the characteristics of the claims. Advantageous additionaldevelopments of the invention are disclosed in the dependent claims.

The invention is based on the concept of only applying the adhesiveannularly in the edge region between the wafer and the substrate ratherthan over a large surface therebetween. This means that the adhesive issituated in a region of the wafer that usually does not contain anystructures. This makes it possible to utilize adhesives that only losetheir adhesive or bonding properties at substantially highertemperatures than those occurring during the rethinning process. Theadhesive used may not lose its adhesive properties until the temperatureexceeds approximately 400° C., namely without destroying the alreadyexisting circuits and contacts. In this case, only the annular edgeregion needs to be heated in order to separate the combination of thesubstrate and the wafer such that structures in the region within theannular adhesive layer are not heated or at least not excessively heatedand therefore tolerate the separation process without damages. Suitableadhesives are based on polymers or epoxy resins.

A particularly sound connection between the substrate and the wafer isproduced if a closed ring of adhesive is applied along thecircumference. Since wafers usually consist of circular structures, itis advantageous to apply the adhesive annularly so as to optimallyutilize the available adhesive surface. The width B of the annularadhesive layer is preferably small in relation to the diameter D of theadhesive layer. The width B typically lies between 2 mm and no more than5 mm.

In the rethinning of wafers that are already provided with contactingterminals, particularly pumps, it is important to mechanically supportthe regions of the wafer that are not fitted with contacting terminalsbecause fluctuations in thickness may otherwise occur during therethinning process due to the deformation of the crystalline structureof the wafer. The invention therefore proposes to provide an elasticfilm in the region enclosed by the annular adhesive layer. Thecontacting terminals protrude into the elastic film and are enclosedthereby. The regions without contacting terminals are mechanicallysupported such that the wafer cannot deflect during the rethinningprocess.

According to an additional development of the invention, it may beadvantageous to use elastic films that are provided with an adhesivelayer on one or both sides in order to additionally improve the adhesionbetween the substrate and the wafer. However, this adhesive layer shouldalready lose its adhesive properties at lower temperatures than theannular adhesive layer in the outer region because excessively hightemperatures would otherwise be required in order to separate the filmand these temperatures could already lead to damages of the structuredwafer. However, no problems arise if the adhesive layer of the filmalready dissolves during the rethinning process because the substrateand the wafer remain fixed to one another by means of the annularadhesive layer.

Another subject of the invention consists of a device for separating theinventive combination of a wafer and a substrate. In this respect, it isdecisive that the substrate and the wafer are separated by heating onlythe region of the combination of the substrate and the wafer that isprovided with the annular adhesive layer. According to the invention,the heating of this edge region is realized with a heating element, theaxial annular heating section of which protrudes in the direction of thesubstrate. In this case, the diameter and the width of the heatingsection at least approximately correspond to the diameter and the widthof the annular adhesive layer. An adjusting device is furthermoreprovided in order to adjust the heating element relative to thecombination of a wafer and a substrate. Since the wafer usually lies onthe substrate, it is advantageous that the heating element can beadjusted perpendicular to the combination of the substrate and thewafer.

In order to ensure that the wafer can be lifted off the substrate afterthe adhesive layer is neutralized, an additional development of theinvention proposes to provide a fixing device for fixing the waferwithin the annular heating section. This fixing device preferablyconsists of a vacuum-based device and holds the wafer by means ofsuction.

In order to ensure that the annular heating section can be lifted offthe wafer despite the wafer being fixed on the fixing device, anadvantageous additional development of the invention proposes that theannular heating section and the fixing device can be adjusted relativeto one another in the axial direction. The annular heating section canbe adjusted, in particular, away from the wafer in the axial directionwhen the wafer is fixed on the fixing device.

According to one solution for realizing the relative adjustabilitybetween the annular heating section and the fixing device that isparticularly advantageous with respect to constructive considerations,the fixing device is suspended on the heating element or anotherstructural element of the device by means of at least one spring. Whenthe heating element is displaced upward together with its annularheating section, the fixing device expands the springs such that thewafer is suspended on the fixing device a certain distance from theannular heating section.

Another subject of the invention consists of a method for handling asubstrate and a wafer. The wafer is fixed on the substrate by applyingan adhesive layer on the substrate and/or the wafer. It is advantageousto apply the adhesive layer on the substrate fixed on the fixing device.According to the invention, it is proposed to apply the adhesive layerannularly. The diameter of the annular adhesive layer should have suchdimensions that it is situated between the substrate and the wafer inthe edge region of the latter. The outside diameter of the adhesivelayer approximately corresponds to the diameter of the wafer.

In substrates with contacting terminals, it is advantageous to apply anelastic film on the substrate in the region within the annular adhesivelayer before the wafer is connected to the substrate, wherein thecontacting terminals are able to protrude into this elastic film. Inorder to realize a thin adhesive layer, it is advantageous to set thesubstrate and/or the wafer in rotation before they are connected, namelyin dependence on which component carries the adhesive layer. Part of theapplied adhesive can be thrown outward due to this measure.

The adhesive layer needs to harden after the wafer and the substrate areconnected. It is advantageous to utilize an adhesive that can behardened by means of heating. However, it would also be possible to use,for example, UF-hardening adhesives.

According to an additional development of the invention, the wafer andthe substrate are separated by heating the annular adhesive layer to apredetermined temperature. At this temperature, the adhesive loses itsadhesive properties such that the wafer can be separated from thesubstrate. It should be observed that only the region of the annularadhesive layer is heated so as to avoid damaging the structures of thewafer. A heating element with an annular heating section is particularlysuitable for heating the adhesive layer.

Other advantages and practical embodiments are disclosed in theremaining claims, the description of the figures and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In these drawings:

FIG. 1 shows a device for connecting a wafer to a substrate;

FIG. 2 shows a substrate with a film applied thereto that is fixed on afixing device;

FIG. 3 shows the substrate according to FIG. 2 with an adhesive layerannularly applied thereon;

FIG. 4 shows a wafer held on a receptacle device shortly before it isconnected to the substrate featuring the annular adhesive layer and thefilm;

FIG. 5 shows a combination of a substrate and a wafer in which thesubstrate is held on a receptacle device and the wafer is held on afixing device;

FIG. 6 shows a device for separating the combination of a wafer and asubstrate;

FIG. 7 shows a combination of a substrate and a wafer in which thesubstrate is held on a fixing device;

FIG. 8 shows an intermediate step during the separation of thecombination of a substrate and a wafer, in which a heating element withan annular heating section is lowered on the wafer and a fixing devicethat is suspended on the heating element by means of springs is situatedin the region within the annular heating section, and

FIG. 9 shows the heating element that can be displaced relative to thefixing device, as well as the wafer that is lifted off the substrate andfixed thereon.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

In the figures, identical components and components with identicalfunctions are identified by the same reference symbol.

FIG. 1 shows a device for connecting a wafer 15 with a substrate 6. Thedevice comprises a chamber 1 with a cover 2. The cover 2 serves fortightly closing the chamber 1 relative to the surroundings. The cover 2simultaneously serves for loading the chamber 1 with the substrate 6 andthe wafer 15. The chamber 1 can be evacuated by means of a not-shownvacuum pump and a vacuum line 3 so as to prevent air inclusions when theconnection between the substrate 6 and the wafer 15 is produced.

A receptacle device 4 (chuck) for accommodating the substrate 6 isprovided within the chamber 1. The receptacle device is connected to amotor unit 5. The receptacle device 4 can be set in rotation by means ofthe motor unit 5.

The device furthermore comprises an adjustable nozzle 7 for applying thefluid adhesive on the substrate 6. Suitable adhesives are based onpolymers or epoxy resins. The receptacle device 4 rotates while as theadhesive is ejected from the nozzle 7 such that the adhesive is appliedin the form of an annular adhesive layer 8 in the edge region betweenthe substrate 6 and the wafer 15.

Vacuum grooves 9 are arranged in the surface of the receptacle device 4in order to prevent the substrate 6 from shifting during the rotation ofthe receptacle device 4, wherein these vacuum grooves make it possibleto fix the substrate 6, in this case also a wafer, on the receptacledevice 4. The receptacle device 4 furthermore features a helical heatingelement 10 in order to quickly harden the adhesive layer 8 that has anannular shape in this case. In the embodiment shown, the helical heatingelements are distributed over the entire surface of the receptacledevice 4 because an adhesive film 14, the adhesive of which is alsohardened, is additionally utilized in this case as described in detailbelow. The adhesive of the film is hardened with the aid of the helicalheating element 10. However, the film 14 can also be omitted,particularly if the wafers do not contain any contacting terminals 17.The vacuum connection of the vacuum grooves 9 as well as the electricalconnections leading to the helical heating elements 10 are notillustrated in the figure.

In addition, a lifting device 11 is provided in the chamber 1 in orderto adjust a fixing device 12 vertically in the direction of thereceptacle device 4. Additional helical heating elements 13 areintegrated into the fixing device 12 in order to maintain the fixingdevice 12 and therefore the wafer 15 at a certain temperature. Therequired temperature sensors and electrical connections of the helicalheating element(s) 13, as well as a control device coupled to thetemperature sensors, are not illustrated in this figure.

FIG. 2 shows the receptacle device 4 with the substrate 6 that lies andis fixed thereon. The aforementioned elastically deformable film 14 isplaced on the substrate 6. In this embodiment, both sides of the film 14are provided with an adhesive layer in order to enable the substrate 6to adhere to the wafer 15. As mentioned above, it is not absolutelyimperative to provide this adhesive layer.

The substrate 6 is introduced into the chamber 1 by means of a not-shownrobot (arm) while the cover 2 is opened, wherein the substrate is placedon the receptacle device 4 and fixed thereon with the aid of the vacuumgrooves 9.

In FIG. 3, an annular adhesive layer 8 is already applied in the edgeregion. The fluid adhesive is ejected from the nozzle shown in FIG. 7that needs to be moved radially outward from the position shown for thispurpose. In order to produce the annular adhesive layer 8, thereceptacle device 4 needs to be set in rotation by means of the motorunit 5. The applied adhesive layer can be partially thrown off byincreasing the rotational speed such that a very thin adhesive layer 8is produced.

In FIG. 4, the wafer 15 is fixed on the fixing device 12 and heldthereon by means of vacuum grooves 16. The fixing device 12 with thewafer 15 fixed thereon is arranged a certain distance from the substrate6 featuring the film 14. In this state, the cover 2 is closed and thechamber 1 is evacuated via the vacuum line 3. The fixing device 12 ispromptly displaced in the direction of the receptacle device foraccommodating the substrate 6 by means of the lifting device 11 andpressed against the substrate with a predetermined pressing force.During this process, contacting terminals 17 provided on the wafer 15are pressed into the elastic film 14 and enclosed thereby. The dispensedadhesive layer 8 is simultaneously compressed between the wafer 15 andthe substrate 6. The adhesive layer 8 is hardened with the aid of thehelical heating elements 10 and 13.

In FIG. 5, the substrate 6 and the wafer 15 already are rigidlyconnected to one another. The combination of the substrate 6 and thewafer 15 is removed by means of a robot after the cover 2 is opened anddelivered to a rethinning process. After the rethinning process, thesubstrate 6 and the wafer 15 need to be separated from one anotherwithout destroying the wafer 15 or the substrate 6. The substrate 6 isreused after this separation process.

FIG. 6 shows a separating device for separating the wafer 15 from thesubstrate 6 that features a separating chamber 18. The separatingchamber 18 is provided with a cover 19 for loading the combination ofthe substrate 6 and the wafer 15 into the separating chamber 18 by meansof a not-shown robot. The separating chamber 18 can be evacuated bymeans of a not-shown vacuum pump and the corresponding vacuum line 22 inorder to create a vacuum or gas atmosphere. A fixing device 20 isprovided within the separating chamber 18 in order to fix the substratewith the aid of vacuum grooves 21. A lifting device 23 is also providedin the separating chamber 18. A trough-shaped heating element 24 isfixed on the lifting device 23. The heating element 24 features anannular heating section with helical heating elements 28, wherein theannular heating section (circumferential wall of the trough) extendsaxially in the direction of the fixing device 20 with the substrate 6and the rethinned wafer 15. A fixing device 25 that is not heated andfeatures vacuum grooves 27 for fixing the wafer 15 is arranged radiallywithin the annular heating section. In the embodiment shown, the fixingdevice 25 has a circular cross-sectional surface. The fixing device 25is suspended on the heating element 24 with the aid of springs 26 thatlimit the displacement of the fixing device. The annular heating sectionand the fixing device 25 can be axially moved relative to one anotherdue to this measure. The width of the annular heating sectionapproximately corresponds to the width of the adhesive layer 8.

The annular heating section is heated to a predetermined temperaturewith the aid of the helical heating elements 28. The temperature ischosen such that the adhesive layer 8 dissolves or the adhesiveproperties of the adhesive used are diminished or neutralized. Theelectrical connections and the required temperature sensors and controllines, as well as the control devices for adjusting the temperature ofthe annular heating section, are not illustrated in the figure.

FIG. 7 shows the combination of the substrate 6 and the wafer 15 in theposition in which they are deposited on the fixing device 20 by means ofa robot. The substrate 6 is fixed in position by means of a vacuumapplied via the vacuum grooves 21. Subsequently, the lifting devicelowers the heating element 24 on the wafer 15 together with the fixingdevice 20. The lowered position is illustrated in FIG. 8. Since only theannular heating section is heated with the aid of the helical heatingelements 28 and the fixing device 25 remains unheated, heat is onlysupplied to the edge region of the rethinned wafer 15 in order toneutralize the adhesive properties of the adhesive. The wafer 15 is notdamaged because the remaining region of the wafer 15, i.e., the regionwithin the annular adhesive layer 8, is not heated or not heated asintensely as the edge region. This means that adhesives can be used thatonly lose their adhesive properties at very high temperatures.

After an adjustable time period that is required for neutralizing theadhesive properties of the adhesive under the influence of heat, therethinned wafer 15 held on the fixing device 25 with the aid of thevacuum grooves 27 is lifted off. This lifting process is illustrated inFIG. 9. This figure shows that the heating element 24 is lifted beforethe fixing device 25. This is caused by the spring suspension. Due tothe springs 26, the heating element 24 with its annular heating section24 is displaced axially relative to the fixing device 25 such that thewafer 15 is held on the fixing device 25 a certain distance from theannular heating section. The rethinned wafer 15 and the substrate 6 arenow removed through the opened cover 19 by means of a robot.

If the wafer 15 does not contain any contacting terminals 17, theelastic film 14 can be omitted. In this case, the annular adhesive layer8 is largely thrown off due to the high-speed rotation of the receptacledevice 4 such that a very thin adhesive layer 8 is produced.

It would also be conceivable to omit the process of lifting off thewafer with the aid of the fixing device 25. It is possible to completelyremove the combination of the substrate 6 and the wafer 15 by means ofthe annular heating section after the neutralization of the adhesivelayer 8 and the transport of this combination to a laminating station,in which the additional processing takes place. The rethinned wafer 15is fixed on a carrier film in this station.

Having described the invention, the following is claimed:
 1. A methodfor preparing a substrate for temporarily bonding to a wafer, the methodcomprising the steps of: applying a first adhesive layer on thesubstrate and/or the wafer, wherein the first adhesive layer is appliedin such a way that the first adhesive layer is disposed annularlybetween the substrate and the wafer in the edge region of the wafer andapplying a second adhesive in the region situated within the annularfirst adhesive layer, wherein said second adhesive loses its adhesiveproperties at a temperature that is lower than a temperature at whichthe first adhesive layer loses its adhesive properties, and wherein theforegoing steps are performed in a manner such that the substrate isthereby configured to temporarily bond to the wafer and configured sothat a treatment of only the annular edge region of the substrate and/orthe wafer is required to reduce the adhesion of the first adhesive layerin order to separate the substrate and the wafer.
 2. The methodaccording to claim 1, wherein the first adhesive layer is applied with awidth of the annular ring between 2 mm and 5 mm.
 3. The method accordingto claim 1, comprising the further step of: fixing the wafer on thesubstrate in that the wafer is applied on the substrate.